Change back to a-tag

Has to be an `<a>` with an empty `href` for the ng-click to fire.

src/Umbraco.Web.UI.Client/src/views/propertyeditors/nestedcontent/nestedcontent.html

@@ -37,9 +37,9 @@
         </div>
 
         <div class="umb-nested-content__footer-bar" ng-hide="nodes.length >= maxItems">
-            <button type="button" class="umb-nested-content__icon" ng-click="openNodeTypePicker($event)" prevent-default>
+            <a href class="umb-nested-content__icon" ng-click="openNodeTypePicker($event)" prevent-default>
                 <i class="icon icon-add"></i>
-            </button>
+            </a>
         </div>
 
     </ng-form>